This invention relates to streamer handling apparatus for use on seismic survey vessels, and is more particularly concerned with such apparatus for use on seismic survey vessels of the kind used in performing 3D marine seismic surveys covering large areas.
In order to perform a 3D marine seismic survey, an array of marine seismic streamers, each typically several thousand meters long and containing a large number of hydrophones and associated electronic equipment distributed along its length, is towed at about 5 knots behind a seismic survey vessel, which also tows one or more seismic sources, typically air guns. Acoustic signals produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophones in the streamers, digitised and then transmitted to the seismic survey vessel, where they are recorded and at least partially processed with the ultimate aim of building up a representation of the earth strata in the area being surveyed.
At the present time, a typical streamer array used by the Applicant comprises a 700 meter wide array of eight evenly spaced streamers, each about 4000 meters long. The streamers are towed by their lead-ins, ie the armoured electrical cables that convey electrical power, control and data signals between the vessel and the streamers, as described in the Applicant""s U.S. Pat. No. 4,798,156, and their spread is controlled and maintained by MONOWING deflectors of the kind described in the Applicant""s U.S. Pat. No. 5,357,892. Wider arrays containing even more streamers can be achieved using the technique of the Applicant""s PCT Patent Application No. PCT/IB98/01435 (WO 99/15913).
From time to time, it may be desired to change the length of one or more of the streamers in an array while at sea. Since each streamer is normally made up from a large number of substantially identical 100 meter streamer sections connected together end to end, such a change is typically achieved by transferring sections from one streamer to another. This typically involves partially reeling in one streamer until only the sections to be transferred are still in the water, disconnecting those sections and connecting them to a derrick, and using the derrick to transfer the disconnected sections to a different streamer station. This procedure has the disadvantages of involving several crew members, and of being time consuming and inconvenient. It is therefore an object of the present invention to provide streamer handling apparatus which alleviates at least one of these disadvantages.
According to the present invention, there is provided streamer handling apparatus for use on a seismic survey vessel, the apparatus comprising guide means adapted to be mounted on the streamer deck of the vessel, adjacent the stern end of the streamer deck, so as to extend transversely over at least a part of the width of the streamer deck, and trolley means movable across the streamer deck, the trolley means being adapted to engage the guide means so as to be guided thereby in its movement across the streamer deck, and including a clamping device for gripping the inboard end of part of a streamer whose other end is deployed over the stern end of the streamer deck.
Thus when it is desired to transfer part of a first streamer to the end of a second streamer, to increase the length of the second streamer, and assuming that the second streamer is initially fully reeled in, the trolley means is positioned at the deployment station of the first streamer, and the first streamer is reeled in over the trolley means until the inboard end of the first streamer section of the part to be transferred reaches the trolley means. The inboard end of that section is then clamped in the clamping device, so that the trolley means and its clamping device take up the load of the part of the streamer to be transferred (which is still deployed over the stern of the streamer deck), and the remainder of the first streamer is disconnected from the part to be transferred. The trolley means, and the end of the streamer part clamped to it, are then moved laterally until they reach the deployment station of the second streamer, whereupon the end of the second streamer is connected to the end of the streamer part clamped to the trolley means. At this point, the clamping device is released, and the second streamer, now of increased length, can be fully deployed.
In a preferred embodiment of the invention, the apparatus comprises two such trolley means, each movable transversely over substantially the whole of a respective half (port or starboard) of the width of the streamer deck.
Advantageously, the guide means includes, for the or each trolley means, a respective guide rail adapted to be secured to but spaced above the streamer deck, and the or each trolley means includes at least one guide wheel which engages the underside of the guide rail, and preferably two such guide wheels which are spaced apart in the direction of movement of the trolley means.
Preferably the or each guide rail is disposed between its trolley means and the stern end of the streamer deck.
Typically, the stern end of the streamer deck is provided with a guard (or safety) rail extending its whole width at about waist height, in which case the or each trolley means may include at least one guide roller which engages the inboard side of the guard rail, and preferably two such guide rollers which are spaced apart in the direction of movement of the trolley means.
Conveniently, the or each trolley means is also provided with at least one support wheel which runs on the streamer deck.
Preferably, the or each trolley means is motor driven, for example by means of a respective hydraulic motor which is fixedly disposed at one end of the guide rail of the trolley means and which drives the trolley means via a chain connected to the trolley means.
Conveniently, the or each trolley means includes storage space, preferably in the form of at least one drawer, for storing tools and the like.
The invention also includes a seismic survey vessel incorporating streamer handling apparatus as defined in any of the preceding statements of invention.